1. Field of the Invention
The present invention relates to an electrochromic device capable of uniform coloring.
2. Related Background Art
A phenomenon of reversible coloration by reversible electrolytic oxidation or reduction under voltage application is called electrochromism.
Various attempts have been made, since more than 20 years ago, to prepare electrochromic devices (ECD) utilizing an electrochromic material showing such electrochromic phenomenon and capable of coloration and color erasure by voltage application, and to utilize such ECD for a light control device such as an anti-glare mirror, or a 7-segment numeric display unit.
For example, U.S. Pat. No. 3,829,196 discloses a totally solid-state ECD composed of a transparent electrode film (cathode), a tungsten trioxide film, an insulating film, for example of silicon dioxide, and an electrode film (anode) laminated in succession on a glass substrate.
The tungsten trioxide (WO.sub.3) film is colored blue when a voltage is applied to said ECD, and returns to the colorless state when an inverse voltage is applied. The mechanism of the coloration and color erasure is not fully understood, but it is theorized that the coloration and color erasure of WO.sub.3 is governed by a small amount of water present in the WO.sub.3 film and the insulating film (ion conductive layer).
The reaction formulae are theorized as follows: EQU H.sub.2 O.fwdarw.H.sup.+ +OH.sup.-
(WO.sub.3 film: cathode) WO.sub.3 +nH.sup.+ +ne.sup.- .fwdarw.H.sub.n WO.sub.3 colorless, transparent colored PA1 (insulating film: anode) OH.sup.- .fwdarw.1/2H.sub.2 O+1/4O.sub.2 .uparw.+1/2e.sup.-
Also, there is already known an ECD composed of an electrochromic layer capable of coloration by reduction (for example WO.sub.3), an ion conductive layer, and a layer capable of reversible electrolytic oxidation (for example, iridium oxide or iridium hydroxide) laminated in succession between an upper electrode and a lower electrode for applying a predetermined voltage.
At least one of the electrode layers directly or indirectly sandwiching the electrochromic layer has to be transparent in order to show the coloration and color erasure to the exterior, and both electrode layers have to be transparent in case of a transmissive ECD.
It is already known that a transparent electrode can be prepared from SnO.sub.2, In.sub.2 O.sub.3, ITO (SnO.sub.2 -In.sub.2 O.sub.3 mixture) or ZnO, for example, but these materials are of relatively low transparency and have to be made thin. Because of this fact, and also because of other considerations, the ECD is usually formed on a substrate such as a glass plate or a plastic plate.
Also, for certain applications, a sealing substrate for protecting the device is positioned opposite to the substrate of the device, and the device is sealed with epoxy resin, for example.
The conventional ECD's have been subject to a significant drawback in that the coloration is very slow and is not uniform, and said uneven coloration is particularly pronounced in a large-sized ECD.